1. Field of the Invention
The present invention relates in general to a device for measuring a displacement of a movable member relative to a fixed member, and more particularly to a device for measuring the displacement by using the electromagnetic induction phenomenon. More specifically, the present invention is concerned with a device for detecting a liquid level in a container by putting the electromagnetic induction phenomenon into practical use.
2. Description of the Prior Art
As one of the conventional liquid level detecting devices, there has been proposed such a device as disclosed in FIG. 1 of the attached drawings. The device "A" disclosed is of a so-called "variable resistance type" arranged in a container in which a liquid to be measured is contained.
The device "A" comprises a zig-zag resistor pattern "B" and an elongate conductive pattern "C" which are arranged in parallel and printed on a rectangular insulating base plate "D". The base plate "D" is fixed in the container to extend vertically. A float E is disposed about the base plate "D" so as to be vertically movable relative to the fixed base plate in accordance with a rise and fall of the level of the liquid in the container. Two conductive sliders "F.sub.1 " and "F.sub.2 " are carried by the float "E" with their leading ends slidably contacting with the two patterns "B" and "C", respectively. The two patterns "B" and "C" and the sliders "F.sub.1 " and "F.sub.2 " thus constitute a variable resistor the resistance of which changes in accordance with the vertical movement of the float "E", that is, in accordance with the fluctuation of the liquid level in the container. The zig-zag pattern "B" (viz., the measuring resistor) is connected to an electric power source "G" through a fixed resistor "H". The voltage fluctuation thus appearing between the measuring resistor "B" and the fixed resistor "H" due to the fluctuation of the liquid level is detected by a voltage detecting circuit "I" and the voltage fluctuation thus detected is treated by an indicating circuit "J" to indicate the amount of the liquid in the container.
However, the above-mentioned device "A" has suffered from the drawback that due to the mechanical contact between each slider "F.sub.1 " or "F.sub.2 " and the printed pattern "B" or "C", long use of the device induces remarkable wear of the measuring resistor "B" changing the original resistance value of the same. This causes erroneous measuring of liquid level. In the severest case, the measuring pattern "B" is broken because of the wear.
Furthermore, due to a friction inevitably produced between each slider "F.sub.1 " or "F.sub.2 " and the printed pattern "B" or "C", the upward or downward movement of the float "E" is not smoothly carried out thereby causing erroneous indication of liquid level. This undesirable phenomenon becomes severer when foreign matter gets in between the sliders and the printed patterns accidentally.